Signaling and procedures for fast and low overhead handover for mu mimo systems

ABSTRACT

A User Equipment (UE) measures the signal quality of a dedicated downlink reference signal transmitted by a Base Station (BS) to the UE. If the signal quality is below a threshold, the UE initiates a measurement procedure to determine a target handover BS. The UE sequentially transmits an uplink reference signal on each of UE&#39;s beams. The BSs estimate the Angle of Arrival (AoA) of any received uplink reference signal, form a dedicated downlink beam pointed toward the AoA and transmit an acknowledgment to the UE on the corresponding beam. The UE estimates the signal quality of the received acknowledgements on the different UE beams, and transmits a handover message on the UE beam on which the acknowledgement with the highest signal quality is received. The uplink reference signal may comprise of a single preamble. The acknowledgement may comprise of a preamble, or a preamble followed by a small payload.

CROSS-REFERENCE

This application claims priority to co-owned and co-pending U.S.application Ser. No. 16/900,466 filed Jun. 12, 2020 entitled:“UPLINK-INITIATED FAST AND LOW OVERHEAD INITIAL ACCESS FOR MU MIMOSYSTEMS”, the contents of which are incorporated by reference in theirentirety.

COPYRIGHT

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent files or records, but otherwise reserves all copyrightrights whatsoever.

FIELD OF THE INVENTION

The present disclosure describes aspects of large MU-MIMO systemscomprising a network of Base Stations (BSs) to provide wirelessbroadband internet access to User Equipment (UEs). The presentdisclosure describes systems and methods to achieve low delay and lowoverhead initial network access, low delay and low overhead active andidle-mode handover to maintain high connection reliability, low overheadand accurate idle mode beam tracking, accurate UE and BS beam steering,and joint optimization of multiple beams to enhance network throughput.

BACKGROUND OF THE INVENTION

High speed and low delay wireless broadband internet access to UserEquipment (UEs) such as smart phones, communications devices forvehicles, and equipment for fixed wireless communications to premisessuch as houses and enterprises has recently gained attention. Frequencybands from below 7 GHz to mmwave range are being considered for 5Gsystems and beyond.

5G and future wireless broadband systems aim at providing data rates ofas high as 1 Gbps or higher, with low delay and high reliability. Suchhigh data rates are achieved using large Multi-User Multiple InputMultiple Output (MU-MIMO) systems, wherein the BSs form many narrowbeams, and each beam is pointed toward at least one UE. These beams havenarrow beamwidth (BW) and require joint optimization and accuratepointing, to maximize network throughput. Low delay and high reliabilityrequire fast UE initial network access, fast beam assignment andpointing by the BSs and the UEs, fast and accurate beam tracking duringthe data session active and idle modes, and fast active-mode andidle-mode handover. The medium access protocol overhead must also beminimized, while achieving low delay and high reliability.

In conventional cellular systems, such as 3G and 4G, reference signalsand control information are broadcast on downlink shared channels, whichthe UEs use to initiate initial access and carry out active and idlemode handover. The downlink-initiated shared control signaling schemesgenerate high overhead in large MIMO systems with many narrow beams. Inthis disclosure, a class of uplink-initiated signaling protocols aredescribed. The UE initiates initial access, active and idle mode beamtracking, and handover based on uplink signaling and measurements. TheBS sends dedicated control information to the UE on dedicated downlinkbeams; thereby significantly reducing overhead, significantly increasingthe execution speed of the aforementioned processes, and providing highbeam pointing and tracking accuracy.

One technique to achieve high data rate and high network throughput isthe simultaneous reuse of the available spectrum on multiple beamsgenerated by a BS. However, reuse of the available spectrum on multiplebeams may result in cross-beam interference, reducing the data rate onsome beams. This disclosure describes systems and methods for joint beamassignment, beam shaping and beam steering of multiple BS beams, toreduce cross-beam interference in the network.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Some embodiments of the present invention are illustrated as an exampleand are not limited by the accompanying figures. In the followingfigures, where appropriate, similar components are identified using thesame reference label.

FIG. 1A is a graphical depiction of an example of multiple BS fixedbeams in a sector, according to some embodiments.

FIG. 1B is a graphical depiction of an example of multiple BS fixedbeams in a sector, according to some embodiments.

FIG. 1C is a graphical depiction of an example of multiple BS steerablebeams in a sector, according to some embodiments.

FIG. 2A is a graphical depiction of an exemplary antenna aperturestructure, according to some embodiments.

FIG. 2B is a graphical depiction of an exemplary antenna aperturestructure, according to some embodiments.

FIG. 2C is a graphical depiction of an exemplary antenna aperturestructure, according to some embodiments.

FIG. 3 is an exemplary graphical depiction of beamformer geometry,according to some embodiments.

FIG. 4 is a graphical depiction of an example of frequency reuse fordownlink data beams, according to some embodiments.

FIG. 5 is a graphical depiction of an example of downlink dedicatedcontrol channel beams and dedicated data beams, according to someembodiments.

FIG. 6A is a flow chart of an exemplary uplink-initiated initial accessprocedure, according to some embodiments.

FIG. 6B is a flow chart of an exemplary uplink-initiated initial accessprocedure, according to some embodiments.

FIG. 6C is a flow chart of an exemplary uplink-initiated initial accessprocedure, according to some embodiments.

FIG. 6D is a flow chart of an exemplary uplink-initiated initial accessprocedure, according to some embodiments.

FIGS. 7A and 7B are graphical depictions of an example of anuplink-initiated initial access signaling, according to someembodiments.

FIG. 7C is an exemplary uplink-initiated initial access message flow,according to some embodiments.

FIG. 8 is a flow chart of an exemplary uplink-initiated handoverprocedure, according to some embodiments.

FIG. 9 is a flow chart of an exemplary beam update process during idleperiod, according to some embodiments.

SUMMARY

A network of Base Stations (BSs) provide broadband internet connectivityto UEs. The BSs and the UEs comprise of at least one radio subsystemconnected to at least one antenna aperture; each antenna aperture iscapable of forming at least one beam. The BS with which a UEcommunicates, the serving-BS, transmits a dedicated downlink referencesignal to the UE. The UE measures the signal quality of the dedicateddownlink reference signal and if the signal quality is below athreshold, then the UE initiates a handover procedure.

During handover, the UE sequentially transmits an uplink handover probeon each of the UE's beams, and waits for an acknowledgement from theBSs. All BSs search for the uplink handover probes. Each BS estimatesthe Angle of Arrival (AoA) of a detected uplink handover probe, forms adedicated downlink beam toward the UE using the estimated AoA, and sendsan acknowledgement to the UE on the dedicated downlink beam. The UEmeasures the signal quality of the acknowledgement. In one embodiment,once the UE has transmitted an uplink handover probe on each of itsbeams, then the UE chooses the UE beam on which the acknowledgement withthe highest signal quality is received and sends a handover message onthe said UE beam, requesting handover. In another embodiment, the UEtransmits a handover message on the first UE beam on which anacknowledgement with a signal quality above a threshold is received.

In some embodiments, the acknowledgement comprises of a single preamble,spread using a pseudo-noise sequence. In another embodiment, theacknowledgements comprise of a preamble and a payload carrying the BSs'identification; the UE chooses, as a handover BS, the BS from which theacknowledgement with the highest signal quality is received, and sends ahandover message to the serving-BS, requesting handover to the handoverBS. In another embodiment, the acknowledgements comprise of preamble anda payload carrying the BSs' identification; the UE forms a handovercandidate set of all the BSs from which an acknowledgement with a signalquality above a threshold is received. The UE sends the handovercandidate set, as well as the signal quality of each BS in the set, tothe BS with which the UE is communicating; the BS chooses a handover BS.In a variation of the embodiment, the UE sends the handover candidateset to a central controller; the central controller chooses a handoverBS.

In some embodiments, the UE tracks changes in the UE's orientation andposition location, computes a metric of the changes in the UE'sorientation and position location, and if the magnitude of the metric isabove a threshold then the UE triggers handover and carries out thehandover as described above.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out exemplary embodiments of the invention. Thedescription is not to be taken in a limiting sense but is made merelyfor the purpose of illustrating the general principles of the invention,as the scope of the invention is best defined by the appended claims.Various inventive features are described below that can each be usedindependently of one another, or in combination with other features.

A network of Base Stations (BSs) provide broadband internet connectivityto UEs. Each BS/UE comprises of at least one radio subsystem connectedto at least one antenna aperture, each antenna aperture is furthercomprised of at least one row and at least one column of antennaelements, the radio subsystem is capable of forming multiplesimultaneous beams, and the radio subsystem enables each antennaaperture to form at least one beam within the sector covering theazimuthal and elevation angular view of the antenna aperture. Each BSbeam is assigned to at least one UE. The BSs may be installedterrestrially, on aerial platforms such as drones, or on satellites. Anexemplary terrestrial BS comprises of four antenna apertures to cover360 degrees in azimuth, where each antenna aperture covers a sector of90 degrees in azimuth.

Abbreviations and terminology definitions are described where theyappear first in the disclosure. Herewith, a glossary of theabbreviations and definitions are provided for easy reference:

-   -   AoA: Angle of Arrival of signals at a UE from a BS, or at a BS        from a UE;    -   UE-to-BS-AoA: AoA of the signals received at a BS from a UE,        with respect to a BS reference direction;    -   DLSRS: DownLink Shared Reference Signal—sent by BSs, and used by        UEs for synchronization, channel impulse response estimation,        and signal quality measurement;    -   DLSCCH: DownLink Shared Control CHannel—used by BSs to send        control and system parameter information to UEs within a sector;    -   DLDRS: DownLink Dedicated Reference Signal—sent by a BS to one        UE, and used by the UE for synchronization, channel impulse        response estimation, and signal quality measurement;    -   DLDCCH: DownLink Dedicated Control CHannel—used by a BS to send        control and system parameter information to one UE;    -   ULAP: UpLink Access Probe—transmitted by a UE, and searched by        the BSs to estimate the UE-to-BS-AoA;    -   ULAP-ACK: the acknowledgement sent to the UE by a BS when the BS        detects a ULAP;    -   ARACH: Asynchronous Random-Access CHannel—an uplink random        access channel on which UEs transmit ULAP without synchronizing        to the downlink;    -   ULRP: UpLink RACH Probe—has the same utility as that of ULAP,        but ULRP is transmitted on the RACH;    -   ULRP-ACK—an acknowledgement sent to the UE by a BS when the BS        detects a ULRP;    -   ULCS: UpLink Connection Setup—a message sent by a UE to a BS        requesting to set up a data connection;    -   ULCS-ACK: an acknowledgement sent to a UE by a BS when the BS        receives a ULCS;    -   LTI: Listen Time Interval—a time interval during which a UE        waits to receive a ULAP-ACK/ULRP-ACK before retransmitting the        ULAP/ULRP;    -   ULCM: UpLink Confirmation Message—a message sent by a UE in        response to the ULCS-ACK to complete the connection set up;    -   Serving-BS: the BS with which a UE is communicating;    -   UE-serving-beam: the UE beam on which the UE transmits/receives        to/from a BS;    -   BS-serving-beam: the BS beam on which the BS transmits/receives        to/from a UE;    -   DLTP: DownLink Tracking Probe—periodically transmitted by a BS        to a UE during idle mode to initiate the update process for        UE-to-BS-AoA, BS-serving-beam, and UE-serving-beam;    -   DLTP-PTI: DLTP Periodicity Time Interval—time interval between        periodic transmission of DLTP by BSs during idle mode;    -   DLTP-WTI: DLTP Wait Time Interval—length of time a UE in idle        mode waits to receive DLTP before timing out and initiating idle        handover;    -   ULHP: UpLink Handover Probe—a reference signal transmitted by UE        for uplink signal quality measurement during handover;    -   ULHM: UpLink Handover Message—the message a UE sends to a BS to        initiate handover;    -   STI: Sleep Time Interval—time interval boundaries where a BS may        begin data transmission to a UE during idle mode, and when the        UE wakes up to search for DLTP;    -   neighbor-set: the set of BSs whose propagation loss to a UE is        below a threshold;    -   handover-candidate-set: the set of BSs whose signal quality is        received at a UE above a threshold    -   handover-BS: the BS to which a UE is handed over when handover        procedure is initiated.

Examples of signal quality are received signal strength (RSS), receivedSNR (Signal to Noise Ratio), and received SINR (Signal to Interferenceplus Noise Ratio).

AoA Estimation and Joint Optimization of Multiple BS Beams

In one approach to forming multiple beams within a sector, the sector isdivided into N sub-sectors, the BS antenna aperture is capable offorming at least N beams whose boresights are fixed toward a set ofspecific angular positions, each beam covers one sub-sector asillustrated by FIG. 1A, where N fixed beams 111-1 to 111-N cover theazimuthal angular range of one sector. The beam which provides thehighest gain toward UE 210, beam 111-2 in FIG. 1A, shown by the solidtriangular shape, is used to communicate with UE 210. However, peak gain(boresight) of beam 111-2 is not placed on UE 210, resulting in reducedreceived signal strength at the UE versus a system that steers theboresight of the beam toward the UE. In another beamforming approachthat uses fixed beams, the BS oversamples the number of fixed beams,generating K overlapping beams as shown in FIG. 1B. A subset of theoverlapping beams is used for concurrent communications with NKK UEs. Inthe example of FIG. 1B, beam 111-3 whose boresight is more closelypointing toward UE 210, compared to that of beam 111-2 in FIG. 1A, isused to communicate with UE 210, thereby achieving higher receivedsignal strength at UE 210. In a steerable multibeam beamformingapproach, the boresight of a BS beam is steered toward each UE that iscommunicating with the BS, thereby maximizing the received signalstrength at the UE. FIG. 1C illustrates a case of 3 steered beams 111-1,111-2 and 111-3, whose boresights are at the azimuthal angles θ₁, θ₂,and θ₃ relative to the upper edge of the sector, and are each pointedtoward a different UE, thereby maximizing the received signal strengthat each UE. Therefore, one aspect of beamforming optimization is tosteer the boresight of the BS beam toward the UE with which the BS iscommunicating, thereby maximizing the received signal strength at theUE.

The main lobe of adjacent co-frequency beams, operating on the samefrequency channel, may create significant cross-beam interference, evenwhen the adjacent beams are spaced by as much as a half power BW ormore. The graphically simplified triangular beams 111-j, shown in FIGS.1A, 1B and 1C, do not exhibit the sidelobes of the beams. The beams'sidelobes cause cross-beam interference even among non-adjacentco-frequency beams. Therefore, a second aspect of beamformingoptimization is to jointly shape multiple beams to minimize thecross-beam interference among multiple co-frequency beams, therebyenhancing the SINR received at the UEs and the BSs and increasingspectrum efficiency. In this disclosure, systems and methods aredescribed for the implementation of beamforming algorithms that placethe boresight of a BS/UE beam toward the desired UE/BS while placingnulls, or large attenuations, toward the other UEs/BSs such as tominimize interference to the other UEs/BSs. Henceforth, the beamformingembodiments are described for the BSs. A person of ordinary skill in theart will recognize that the systems and methods described in thisdisclosure for beamforming by BSs also apply to beamforming by UEs,without departing from the scope of the disclosure.

One exemplary BS antenna aperture is comprised of M columns and L rowsof antenna elements. The BS is capable of forming multiple beams andsteering the beams in azimuth and elevation by determining thebeamformer phases and gains for each beam, and applying the phases andgains to each of the antenna elements in the rows and columns of antennaelements. In one approach to beam steering in azimuth and elevation, afactorable 2-dimensional array whose response is the product ofresponses in the azimuth and elevation axes is used. If the beamformercoefficients, w_(lk), where l is the index of the elements in a columnof elements and k is the index of the elements in a row of elements, arewritten as w_(lk)=γ_(l)e^(jθ) ^(l) β_(k)e^(jθ) ^(k) , where {γ_(l)e^(jθ)^(l) }_(l=1) ^(L) are the elevation beamformer coefficients that shapethe beam in elevation, and {β_(k)e^(jθ) ^(k) }_(k=1) ^(M) are theazimuth beamformer coefficients that shape the beam in azimuth. Then,the array response may be written as

H(ω_(x),ω_(y))=Σ_(l=1) ^(L)Σ_(k=1) ^(M) w _(lk) e ^(−j(lω) ^(y) ^(+kω)^(x) ⁾=Σ_(l=1) ^(L)γ_(l) e ^(jθ) ^(l) Σ_(k=1) ^(K)β_(k) e ^(jθ) ^(k) e^(−jkω) ^(x) =H(ω_(x))H(ω_(y)).  (1)

Equation (1) shows that for the factorable 2-dimensional array, thebeamformer may, separately, compute and apply the beamformercoefficients for steering the beam in the azimuth and elevationdimensions.

The BSs/UEs may generate beams using digital beamforming, analogbeamforming or a hybrid of analog and digital beamforming. FIG. 2Aillustrates an exemplary antenna aperture that comprises of rows andcolumns of antenna elements 112-j. In one embodiment, antenna elements112-j are spaced at half wavelength. In one embodiment, digitalbeamforming implementation is used for the application of the azimuthand elevation beamformer coefficients for the antenna aperture of FIG.2A. In another embodiment, digital beamforming is used to apply theazimuth beamformer coefficients, and analog beamforming is used to applythe elevation beamformer coefficients for the antenna aperture of FIG.2A. FIG. 2B illustrates an exemplary antenna aperture comprised ofantenna elements 114-j that are longer in the y-direction (elevation)and are shorter in the x-direction (azimuth). In one embodiment, digitalbeamforming is used to implement the azimuth and elevation beamformercoefficients of the exemplary antenna aperture of FIG. 2B. In avariation of the embodiment, antenna elements 114-j are spaced at halfwavelength in each row.

FIG. 2C illustrates an antenna aperture wherein a number of antennaelements of the type 112-k are grouped in the y-direction to formantenna sub-apertures 116-j. In one embodiment for beamformercoefficient implementation for the exemplary antenna aperture of FIG.2C, digital beamforming is used to implement the azimuth beamformercoefficients for the antenna sub-apertures 116-j, hybrid beamforming isused in the elevation direction wherein the elevation beamformercoefficients are further divided into a product of a first and a secondset of elevation beamformer coefficients, the first set of elevationbeamformer coefficients are applied to the elements 112-k within anantenna sub-aperture 116-j using analog beamforming, the second set ofelevation beamformer coefficients are applied to the antennasub-apertures 116-j using digital beamforming. In a variation of theembodiment, the analog beamformer coefficients applied to the elements112-k within an antenna sub-aperture 116-j are chosen to tilt the beamof each antenna sub-aperture toward a first elevation angle, and theelevation beamformer coefficients applied digitally to the antennasub-apertures 116-j steer the beam in elevation within a range ofelevation angles around the said first elevation angle. Theaforementioned elevation hybrid beamforming embodiment reduces thenumber of required D/A and A/D convertors versus a fully digitalbeamforming scheme; thereby reducing power consumption, while usinganalog beamforming to focus the angular range of the digital elevationbeam steering to the desired range for each BS sector.

Henceforth, systems and methods are described for the implementation ofthe azimuth beamformer coefficients. A person with ordinary skill in theart will recognize that the same systems and methods are also applicablefor implementing the elevation beamformer coefficients, withoutdeparting from the scope of the disclosure. Consider azimuth beamshaping and steering using one row of M antenna elements. FIG. 3 shows abeamformer geometry for an antenna aperture with M antenna elements inazimuth spaced at half a wavelength, the angle of arrival of the signalreceived from a UE, and the difference between the distance that thesignal travels to arrive at two adjacent antenna elements. One exemplaryalgorithm that reduces cross-beam interference is described next. Let w_(k) denote an M by 1 vector of the azimuth beamformer coefficients forthe k-th UE, N be the number of simultaneous beams, H denote the M by Nchannel impulse response matrix, and r _(k)=[r_(k)(θ₁) r_(k)(θ₂) . . .r_(k)(θ_(N))]^(T) be a desired spatial filter response for the k-th UE,where r_(k)(θ_(k)) is the desired spatial filter's response for the k-thuser whose signal is being received at the BS at the angle θ_(k).

In one embodiment, r_(k) (θ_(k)) is set to 1 for the angle toward thek-th UE and to 0 for r_(k)(θi), i≠ k, toward the other N−1 UEs, toeliminate interference to those UEs. One exemplary Least Square (LS)algorithm for optimizing the received SINR at the UEs, also referred toas Zero-Forcing (ZF) algorithm in the literature, computes the spatialfilter weights w _(k) for the k-th UE to minimize

| H ^(H) w _(k) −r _(k)|²,  (2)

The solution to (2) is given by

w _(k) =H ( H ^(H) H )⁻¹ r _(k),  (3)

assuming the inverse exists, where the underlined lowercase bold lettersindicate vectors, and the underlined capital bold letters indicatematrices. There are other variations of the above exemplary ZFalgorithm. A person with ordinary skill in the art will recognize thatthe systems and methods described in this disclosure for theimplementation of beam shaping algorithms are also applicable to thevariations of the above exemplary ZF algorithm, without departing fromthe scope of the disclosure.

In general, for a system with wide frequency band, fast time varyingchannel and a large number of antenna elements M estimating the channelimpulse response matrix H may require high overhead. On the other hand,as M increases, the beam's beamwidth (BW) decreases, resulting in asmaller multipath delay spread and a channel impulse response thatchanges slowly across the frequency band, requiring a smaller number ofchannel impulse response samples for interpolation to estimate the fullchannel impulse response. As an example, consider a system where the BSantenna aperture has a row of M=32 and L=32 antenna elements. At 30 GHz,an antenna aperture with 32 by 32 elements, spaced at half wavelength,has a size of about 16 cm by 16 cm, which is feasible to deploy. The BWof the 32 by 32 element antenna aperture beam is about 3 degrees. Also,consider a UE antenna comprising of a row of four antenna elementsspaced at half wavelength, whose BW is about 45 degrees. Then, for theaforementioned BS and UE antenna BWs and for LOS conditions, multipathis limited to a small area around the receiving antenna which may resultin a flat frequency response. Even in NLOS conditions where delay spreadmay be larger relative to LOS, the channel impulse response is expected,due to the small BS antenna beam BW, to change slowly across thefrequency band.

The array response vector is defined as

α(θ_(i))=[1e ^(jw) ^(s) ^((θi)) . . . e ^(j(M−1)w) ^(s)^((θi))]^(T)  (4)

where w_(s)(θ_(i))=2πΔ sin(θ_(i))/λ, Δ is antenna element spacing, and λdenotes wavelength. If Δ=λ/2, then w_(s)(θ_(i))=π sin(θ_(i)). The arrayresponse matrix is defined as A=[α(θ₁) α(θ₂) . . . α(θ_(N))]. In LOSconditions, where channel frequency response is flat, the channelimpulse response matrix reduces to

H ^(LOS)=[h ₁α(θ₁)h ₂α(θ₂) . . . h _(N)α(θ_(N))],  (5)

where h_(i) is the complex scalar flat frequency response from the BSantenna aperture to the i-th UE. In NLOS conditions with small delayspread, the channel impulse response matrix may be written as

H ^(NLOS)=[h ₁(w)α(θ₁)h ₂(w)α(θ₂) . . . h _(N)(w)α(θ_(N))],  (6)

where h_(i)(w) is the multipath channel impulse response from the BSantenna aperture to the i-th UE.

In one embodiment for the estimation of the DL frequency channel impulseresponse component h_(i)(w), the BS forms a DownLink (DL) beam towardthe UE using an estimate of the AoA at the BS of the signals receivedfrom the UE, the BS transmits a DownLink Reference Signal (DLRS) on asubset of the frequencies separated across the frequency band, the UEdetects the DLRS signals on the said subset of the frequencies, the UEestimates a partial channel impulse response for the said subset of thefrequencies and interpolates the said partial channel impulse responseto estimate the full DL frequency channel impulse response h_(i)(w).

The channel impulse response matrices in (5) and (6) require that allbeamformer hardware transmit paths be calibrated, and all hardwarerelated phases and gains of the different beamformer transmit paths becompensated prior to applying the beamformer coefficients computed basedon the said channel impulse response matrices. Let c_(i) be the hardwarerelated complex scalar response of the i-th transmit path from digitalsamples to upconversion, to power amplification and to transmissionthrough the antenna element. In one embodiment, the i-th row of H, thechannel impulse response for the DL frequency, is multiplied by 1/c_(i)to remove any hardware related phases/gains, prior to applying thespatial filter coefficients for the DL (transmit) beams as computedbased on (3) to (6) and variations thereof.

Let d_(i) be the hardware related phases/gains of the i-th receive pathfrom antenna element to radio front end, to downconversion and tobaseband. In one embodiment, the i-th row of H, the channel impulseresponse for the UL frequency, is multiplied by 1/d_(i) to remove anyreceive path hardware related phases/gains prior to applying the spatialfilter coefficients for UL (receive) beams as computed based on (3) or(6) and variations thereof. In one embodiment for the estimation of theUL frequency channel impulse response component h_(i)(w), the UE forms aUE beam on which the UE receives the highest signal quality from the BSand transmits an UpLink Reference Signal (ULRS) on a subset of thefrequencies separated across the frequency band, the BS estimates apartial channel impulse response for the said subset of the frequenciesand interpolates the said partial channel impulse response to estimatethe full UL frequency channel impulse response h_(i)(w).

In some embodiments, the available frequency band, F, is divided into aset of frequency channels F₁, . . . F_(N). For a given frequency reusepattern, each frequency channel, F_(i), is assigned to a subset of theBS beams. In one frequency reuse pattern, all frequency channels, F_(i),are assigned to all beams, referred to as the frequency reuse of 1,where in effect all the available frequency band is simultaneously usedon each beam. FIG. 4, illustrates a scenario where the BS forms 10beams, the available frequency band is divided into two frequencychannels, both frequency channels F₁ and F₂ are assigned to beams 111-1to 111-6 and to beam 111-10. Beams 111-7 to 111-9, which are highlyoverlapped, are only assigned one frequency channel, wherein theadjacent beams do not use the same frequency channel in order to reducecross-beam interference among the overlapping beams. The optimalfrequency reuse pattern, for a given set of beams, is the one thatachieves maximum total throughput defined as the sum of the data ratesover all beams. The total throughput of a frequency reuse pattern, for aset of beams, depends on the joint beamforming algorithm. In oneembodiment, the beam shaping and frequency reuse pattern determinationfor a set of beams are carried out jointly, wherein: the BS specifies aset of frequency reuse patterns, uses a joint multibeam beamformingalgorithm to optimize the SINR on each set of the co-frequency beams foreach of the frequency reuse patterns, computes the total throughput foreach frequency reuse pattern by summing the data rates over all beamsfor the said frequency reuse pattern, and chooses the frequency reusepattern with the highest total throughput. In a variation of theembodiment, for systems that use dual polarization antenna apertures,each frequency channel F_(i) is used twice, as channels F_(i,1) andF_(i,2), where F_(i,1) is channel F_(i) used on a first antennapolarization, and F_(i,2) is channel F_(i) used on a second antennapolarization, and all frequency channels F_(i,1) and F_(i,2) are used inspecifying a frequency reuse pattern.

In order to implement the aforementioned beamforming schemes, Angle ofArrival (AoA) of the signal received at a BS from a UE, angle θ_(i) inFIG. 3, is needed. The AoA of a signal received at a BS from a UE is, inthis disclosure, referred to as the UE-to-BS-AoA. The BSs form threemain types of dedicated beams for each UE, the UL search beams, the DLdedicated control beams, and DL/UL dedicated data beams. The UL searchbeams are used to search the uplink signals to estimate theUE-to-BS-AoA. The DL dedicated control beams are used to send dedicatedcontrol information to the UE. The DL and UL dedicated data beams areused to send and receive data and control information to/from the UE. Inone embodiment, the DL dedicated control beams and the DL/UL dedicateddata beams that are formed by a BS, and pointed toward a set of UEswithin a sector, are jointly shaped using the estimated UE-to-BS-AoAsfor the said UEs and a joint beamforming optimization algorithmdescribed by equations (3) through (6) or variations thereof.

A class of UEs, such as devices for broadband to homes, may have antennaaperture dimensions comparable to those of BSs, thereby forming beamswith narrow BWs. Furthermore, a reference direction may also be definedfor the fixed UEs, as is done for the BSs. Then, a fixed UE may estimatethe AoA of the signals received from a BS, use the AoA to point theboresight of the UE beam toward the BS with which the UE iscommunicating, while placing nulls at a number of other BSs to minimizeinterference to/from the BSs. In one embodiment, the UE is configuredwith the position location coordinates of the BSs, the UE is alsoconfigured with its own position location coordinates, or estimates itsown position location coordinates using a geo-location technology, andestimates the AoA of the signals received from the BSs using theposition location coordinates of the BSs and the UE.

In one embodiment for estimating the UE-to-BS-AoA, the BS uses digitalbeamforming, the UE transmits an UpLink Access Probe (ULAP) signal, theULAP carries a reference signal to enable the BS to measure the ULAPsignal quality, and the BS carries out a search, for ULAPs, infrequency, time and space by forming and sweeping a UL search beamwithin the BS sector angular range until a ULAP signal is detected. Theinitial UL search beam boresight angular pointing position where theULAP is detected is defined as the nominal angular position. In avariation of the embodiment, the BS forms a set of overlapping beamsthat cover the sector angular range, searches each of the overlappingbeams, and chooses the beam from which the highest signal quality isreceived as the nominal angular position. In one embodiment, the BSiterates the nominal angular position to enhance the estimate of theUE-to-BS-AoA according to: the BS specifies two candidate angularpositions separated from the nominal angular position by ±Δ_(θ)(n)degrees, n being the iteration index, estimates the ULAP received signalquality at each of the two candidate angular positions, and if the ULAPreceived signal quality from a candidate angular position is higher thanthat of the nominal angular position then the BS sets the nominalangular position to that of the candidate angular position with thehighest signal quality, until a specified convergence criterion issatisfied at which time the BS sets the UE-to-BS-AoA to that of thelatest nominal angular position. In a variation of the embodiment, theparameter Δ_(θ)(n) is adaptively made smaller during the differentiterations in order to increase the accuracy of the UE-to-BS-AoAestimate while achieving fast convergence.

In another embodiment for determining the UE-to-BS-AoA, a two-stagespatial search of the ULAP signals, comprising of a coarse spatialsearch stage followed by a fine spatial search stage, is carried out.During the coarse spatial search, the BS divides its sector angularcoverage range into a number of sub-sectors, sequentially forms a ULsearch beam covering each subsector and searches for ULAPs using eachsub-sector beam. If a ULAP is detected on a sub-sector beam, the spatialsearch enters fine mode search, where the BS specifies an angular rangearound the boresight angular position of the UL search beam on which theULAP is detected at the end of the coarse search, divides the saidangular range into a number of smaller fine angular sub-ranges,sequentially forms UL search beams whose boresights point to the centerof each angular sub-range, estimates the ULAP received signal quality oneach of the said UL search beams, chooses the UL search beam with thehighest ULAP received signal quality, and uses the boresight angularposition of the UL search beam as the UE-to-BS-AoA of the signalsreceived at the BS from the UE. In a variation of the embodiment, thefine spatial search stage further comprises of multiple stages, wherethe fine angular sub-range of each subsequent fine spatial search aresmaller than that of the previous fine search.

In one embodiment, once the BS detects a ULAP and determines theUE-to-BS-AoA, the BS forms a DL dedicated control beam toward the UE andtransmits a ULAP-ACK, acknowledging the reception of the ULAP. In oneembodiment, the ULAP comprises of a reference signal that is spread witha specific modulation symbol pattern, to distinguish the ULAP from othersignals, effectively carrying one bit of information, which informs a BSthat a UE is attempting initial access. In this disclosure, themodulation symbol pattern that is used to spread a reference signal isalso referred to as a pseudo-noise sequence. In one embodiment, the ULAPsignal length is chosen to be long enough so that the receiver of theclosest BS accumulates enough energy, after dispreading, to detect theULAP. In one embodiment, the ULAP-ACK also comprises of a referencesignal, spread using a specific pseudo-noise spreading sequence,effectively carrying one bit of information. The ULAP and ULAP-ACKreference signals are also, in this disclosure, referred to as preamble.In a variation of the embodiment, the ULAP-ACK is transmitted onspecific physical layer resources, such as a specific set oftones/symbols in an OFDM based system, for which UE searches aftersending a ULAP. In another embodiment, the ULAP and ULAP-ACK arecomprised of a preamble followed by a payload carrying information suchas the UE/BS identification.

Fast Uplink-Initiated Initial Access

In conventional cellular systems (e.g., 3G and 4G systems), a BSbroadcasts a DownLink Shared Reference Signal (DLSRS) and a DownLinkShared Control Channel (DLSCCH) on a sector-wide beam. During theinitial network access, a UE searches for the DLSRS transmitted by theBSs, estimates the received signal quality of each detected DLSRS,chooses the BS from which the UE receives the DLSRS with the highestsignal quality with which to communicate, and decodes the DLSCCH of theBS to obtain system parameters. A sector-wide broadcast DLSRS/DLSCCHapproach may also be used in MU-MMIO systems. However, antenna aperturesof MU-MIMO systems that are designed to provide a large number of beamsare capable of forming high gain narrow beams, to extend the sectorrange and/or data rate. If a sector-wide beam is used to transmit theDLSRS/DLSCCH, then a UE, which may have a low gain antenna beam, willneed to receive the DLSRS for a long time duration, and receive theDLSCCH at a low data rate, in order to achieve adequate processing gainto successfully detect the DLSRS and decode the DLSCCH messages. LongDLSRS signals, and low data rate DLSCCH messages, will increase thesystem overhead, thereby reducing network throughput. In some systemsthat use beamforming with M antenna elements, where each antenna elementhas a dedicated power amplifier, the transmit EIRP (Effective IsotropicRadiated Power) may be reduced by a factor of M² when only one antennaelement is used to broadcast the DLSRS/DLSCCH within a sector, whichresults in a significant link budget loss, thereby requiring to increasethe length of the DLSRS/DLSCCH and their associated overheard.

As an alternative to a sector-wide beam, narrow beams may be used totransmit DLSRS/DLSCCH. In one embodiment, a sector may be covered by afixed number of beams, equally spaced as shown in FIG. 1A, and theDLSRS/DLSCCH may be transmitted on one beam at a time. Beam 111-2 inFIG. 1A is transmitting DLSRS/DLSCCH, while dotted beams are nottransmitting, wherein the entire BS sector EIRP is dedicated to thesingle transmitting beam, allowing shorter DLSRS and higher data rateDLSCCH. In this approach, the DLSRS/DLSCCH are transmitted multipletimes, once per fixed beam, which also tends to increase the DLoverhead. In another embodiment, DLSRSs/DLSCCHs are concurrentlytransmitted on all narrow beams in a sector. In this approach, theDLSRS/DLSCCH of all narrow beams are transmitted simultaneously, but theavailable BS EIRP is shared among all beams, resulting in a longer DLSRSand lower data rate DLSCCH, versus the one narrow beam at a timeapproach. Furthermore, in the aforementioned DLSRS/DLSCCH transmissionapproaches, the DLSRSs/DLSCCHs need to be transmitted frequently such asto maintain the initial system access delay by the UEs below a certainthreshold. The frequent transmission of DLSRS/DLSCCH will furtherincrease the overhead, thereby reducing network throughput. Henceforth,in this section, an uplink-initiated initial access is described thatachieves fast initial access while significantly reducing signalingoverhead.

The current UE-to-BS-AOA is defined as the latest estimated AoA ofsignals received at a BS from a UE. In one embodiment foruplink-initiated initial access, the UEs begin initial access bytransmitting a ULAP in an asynchronous manner without first acquiring DLsynchronization prior to the ULAP transmission. In one embodiment, theULAP may be sent on an uplink Asynchronous RACH (ARACH) channel whoseresources, in terms of frequency allocation, are a priori known to theUEs, such as having been downloaded to the UEs as configurationparameters. In one embodiment, the UEs use an initial UE configurationphase procedure, wherein the BSs periodically transmit a DLSRS and a DLShared Configuration Message (DLSCM) on a fixed set of narrow beamscovering each sector of a BS, the UEs search for the DLSRS during theinitial UE configuration phase, detect the DLSRS and decode the DLSCM toextract system parameter information such as ARACH frequency resources.Since the initial UE configuration needs to be rarely executed by theUEs, perhaps only once when a user procures and activates the device,then the DLSRS/DLSCM may be periodically transmitted on a set of fixedbeams covering a sector with a small periodicity, perhaps as low as oncea minute resulting in negligible overhead. In another embodiment, asecond network, such as a legacy LTE network, may be used to send theconfiguration parameters to the UEs.

In one embodiment, the ULAP-ACK includes a message that carries systemparameter information such as frequency and time slot resourceallocation of the RACH of the BS which is transmitting the ULAP-ACK, aswell as those of the BSs within a certain range from the BS. Then, theUE after receiving one ULAP-ACK on one UE beam, has synchronized to theDL, will have information on the RACH resources and timing, and maytransmit the ULAPs on the other UE beams on the RACH, thereby reducingload on the ARACH. An access probe that a UE transmits on the RACH isreferred to as the UpLink RACH Probe (ULRP), and the ULRP-ACK is theacknowledgment to the ULRP. The ULRP has the same functionality as thatof the ULAP but is transmitted on the RACH instead of the ARACH. Inanother embodiment, the UE is capable of communicating with a first anda second network, the second network is used to send information, to theUEs, on the RACH resources of the first network. An example of thesecond network is an LTE network. In a variation of the embodiment, thefirst and second networks are synchronized, the UE synchronizes to thesecond network, receives information regarding the RACH resources fromthe second network, and transmits a ULRP, instead of a ULAP, on the UEbeams. Henceforth, the uplink-initiated initial access embodiments aredescribed using the ULAP as the access probe. A person with ordinaryskill in the art will recognize that the systems and methods describedfor the ULAP-based initial access embodiments also apply when the ULRPis used instead of the ULAP, without departing from the scope of theULAP based embodiments.

In one uplink-initiated initial access procedure, a UE initiates initialaccess by sequentially transmitting a ULAP on each of the UE's beams. Aswas described in embodiments of a previous section, the BSs continuouslycarry out a frequency, temporal and spatial search of the ULAP messagesby sweeping narrow UL search beams within their sector coverage area. Ifa BS detects a ULAP, the BS designates the UL search beam angularpointing position on which the ULAP is received with the highest signalquality as the AoA of the ULAP received at the BS from the UE, sets thecurrent UE-to-BS-AOA to the AoA, forms a DL dedicated control beam usingthe current UE-to-BS-AOAs of all the active UEs and a multibeambeamforming algorithm, and transmits a ULAP-ACK to the UE on the DLdedicated control beam. In other words, in the uplink-initiated initialaccess, the DL control signaling to the UEs is sent on DL dedicatedcontrol beams, thereby avoiding the use of DLSCCH in order to minimizeDL overhead. FIG. 5 illustrates a BS that has formed seven DL dedicateddata beams 111-1 to 111-7 shown by the solid lines, and three DLdedicated control beams 112-1 to 112-3 shown by the dashed lines. In oneembodiment, the DL dedicated data beams and the DL dedicated controlbeams are all steered to their respective UEs and are formed using ajoint beamforming scheme to maximize the received SINR on each beam asdescribed in the previous embodiments.

FIG. 6A provides an exemplary flow chart of the UE steps for anuplink-initiated initial access procedure, according to one embodiment.The Listen Time Interval (LTI) is defined as the time interval that theUE waits to receive an acknowledgement from a BS, for a ULAP that the UEhas transmitted, before retransmitting the ULAP. In step 102, the UEtransmits a ULAP on a UE beam on which a ULAP has not yet beentransmitted, sets the LTI timer and begins the search for ULAP-ACKmessages on the same UE beam. In step 104, if a ULRP-ACK has beenreceived within the LTI, then the UE in step 202 estimates the signalquality of the received ULAP-ACK and goes to step 204. If in step 104 aULAP-ACK has not been received with the LTI, then the UE moves to step106. In step 106, if the number of transmissions of the ULAP on the sameUE beam is less than a number N_(T), the UE retransmits the ULAP in step108 and goes back to step 104, otherwise the UE moves to step 204. Instep 204, if ULAPs have not transmitted on all UE beams, then the UEgoes back to step 102 to transmit a ULAP on another UE beam, otherwisethe UE moves to step 206, chooses a UE beam based on the signalqualities of the detected the ULAP-ACKs, and transmits an UpLinkConnection Setup (ULCS) message on the said UE beam initiating aconnection setup procedure. In one variation of the embodiment, the UEchooses the UE beam on which a ULAP-ACK with the highest signal qualityis received. In another variation of the embodiment, the UE increasesthe power transmitted on the ULAP for each retransmission of the sameULAP. In another variation of the embodiment, the ULCS comprises of apreamble and a payload that carries information such as the UE'sidentification. The LTI must be chosen to be larger than the sum of thetransmission time of the ULAP, the round-trip delay from the UE to thefarthest BS that may detect the ULAP transmitted by the UE, the ULAPsignal processing time at the BS, the BS scheduling delay for theULAP-ACK, and the transmission and propagation time of the ULAP-ACK.

A BS that receives a ULCS from a UE, transmits a ULCS-ACK to the UE onthe DL dedicated control beam formed toward the UE using the currentUE-to-BS-AOA, acknowledging the ULCS. The ULCS-ACK comprises of apreamble and a payload that carries information such as the BS'sidentification. In one embodiment, the ULAP-ACKs transmitted by all BSscomprise of only a preamble and use the same spreading code which arenot BS specific. If a UE receives multiple ULAP-ACKs on one UE beam andtransmits a ULCS message on the same beam, then multiple BSs may receivethe ULCS message and multiple BSs may respond with ULCS-ACK messages. Inone embodiment, in order to resolve the ambiguity that may result whenmultiple BSs send a ULCS-ACK to a UE, the UE extracts the BS'sidentification information from each ULCS-ACK, determines the BS fromwhich it receives the ULCS-ACK with the highest signal quality, andtransmits a UL Confirmation Message (ULCM) carrying the UEidentification, thereby completing the connection with only one BS. Inanother embodiment, the ULAP-ACK transmitted by each BS is spread usinga BS specific pseudo-noise sequence, and the UE includes in the ULCS theidentity of the ULAP-ACK pseudo-noise sequence with the highest receivedsignal quality. In another embodiment, the ULAP-ACK comprises of apreamble and a payload carrying the BS's identification, and the UEincludes in the ULCS the identification of the BS from which a ULAP-ACKwith the highest signal quality has been received. In a variation of theembodiments, the BS whose identity is included in the ULCS transmits theULCS-ACK.

In another embodiment, the ULAP-ACKs carry the identification of theBSs; the UE includes in the ULCS the identity of the BSs from which aULAP-ACK has been received as well as the signal qualities of thereceived ULAP-ACKs; the UE transmits the ULCS on one of the UE beams onwhich a ULAP-ACK is received; a BS that receives the ULCS forwards theULCS to a Central Controller (CC); the CC chooses a BS from among theBSs listed in the ULCS with which to establish the connection with theUE, and sends a request to the BS to set up the connection; and the BSsends a ULCS-ACK message to the UE establishing connection. The CCfunctionality may be supported by the processor subsystem of one of theBSs, or by a dedicated processor.

If multiple BSs receive the same ULAP and transmit their ULAP-ACKswithin the same time slot using the same frequency resources, then themultiple ULAP-ACKS will collide. In one embodiment for avoiding ULAP-ACKcollision, different ULAP-ACK time slot scheduling delays are assignedto a set of BSs to stagger, in time, the ULAP-ACKs transmitted by theBSs. In another embodiment for avoiding collision, different frequencyresources are assigned to a set of BSs for the transmission of theULAP-ACKs.

FIG. 6B provides an exemplary flow chart of the UE steps for anuplink-initiated initial access procedure, according to one embodiment.In step 102, the UE transmits a ULAP on a UE beam on which a ULAP hasnot yet been transmitted, sets the LTI timer and begins the search forULAP-ACK messages on the same UE beam. In step 104, if a ULRP-ACK hasbeen received within the LTI, then the UE moves to step 202. In step202, the UE estimates the signal quality of the received ULAP-ACK, andif the signal quality is above a threshold then the UE goes to step 208to transmit a ULCS on the same UE beam, otherwise the process goes tostep 204. If step 104 indicates that a ULAP-ACK has not been receivedwithin the LTI, the process moves to step 106. In step 106, if thenumber of transmissions of the ULAP on the same UE beam is less than anumber N_(T), then the UE, in step 108, retransmits the ULAP and goes tostep 104, otherwise the process moves to step 204. In step 204, if ULAPshave not been transmitted on all UE beams, then the UE goes back to step102 to transmit a ULAP on another UE beam, otherwise the UE goes to step210 where the process ends. In a variation of the embodiment, the UEincreases the power transmitted on the ULAP for each retransmission ofthe same ULAP.

In one embodiment, a UE forms a handover-candidate-set as a set of BSsfrom which a ULCS-ACK with a signal quality above a threshold isreceived. A handover-BS is the BS to which a UE is handed over when theUE initiates a handover procedure. As will be described later, when ahandover procedure is initiated, a UE or a CC (Central Controller) mayuse the UE's handover-candidate-set as the set of BSs which the UE orthe CC may evaluate to determine the handover-BS.

FIGS. 7A, 7B and 7C illustrate an example of an uplink-initiated initialaccess as described above, where the UE is capable of forming one of twobeams at a time. As illustrated in FIG. 7A, UE 210 initially forms beam242-1, shown by the dashed semicircle, and transmits ULAP message 232-1,shown by the dashed zig-zag line, on beam 242-1. BSs 220-1 and 220-2determine the UE-to-BS-AOAs using the UL search beam angular position onwhich they receive ULAP 232-1 with the highest signal quality. The BSsthen transmit ULAP-ACK messages 233-1 and 233-2, shown by the solidzig-zag lines, to UE 210 on beams 240-1 and 240-2 formed using theUE-to-BS-AOAs. Upon receiving the ULAP-ACKs on beam 242-1, the UEtransmits a ULAP on its second beam 242-2, as shown in FIG. 7B. OnceULAP-ACK 233-3 has been received for ULAP 232-2, the UE chooses theULAP-ACK with the highest received signal quality, and sends a ULCS onthe same UE beam from which the said ULAP-ACK was received, initiatingconnection setup. The BS, after receiving the ULCS from the UE,transmits a ULCS-ACK to the UE on a DL dedicated control beam that isformed using the current UE-to-BS-AOA. In one embodiment, the BStransmits the ULCS-ACK on the same DL dedicated control beam on whichthe ULAP-ACK was transmitted. In a variation of the embodiment, the BSalso informs the CC of the established connection between the BS and theUE.

FIG. 7C illustrates an exemplary timeline and sequence of messagesexchanged between the UE and the three BSs in the example of FIGS. 7Aand 7B. In FIG. 7C, the time from the beginning of each arrow to the tipof the arrow is the transmission plus propagation time for each message.The time between the reception of a message and the start oftransmission of a response by the BS accounts for signal processing andDL scheduling delay at the BS. Therefore, the total initial access delayfor a UE with multiple beams and a BS employing digital beamforming maybe approximated by summing all the ULAP transmission/propagation times,the signal processing/transmission/propagation times and DL schedulingof the ULCS-ACKs, all the timed out LTIs, the ULCStransmission/propagation times, and the signalprocessing/transmission/propagation times and DL scheduling of theULCS-ACKs. The propagation delay is of order of micro-seconds forsectors with radius of a few kms. In systems with data rates of tens ofMbps or higher, the transmission times of the ULAP and the ULAP-ACKmessages may be of order of tens to hundreds of micro-seconds becausethese messages carry a small amount of information, as small as only asingle bit of information. Then, the ULAP-ACK DL scheduling andtransmission/propagation times, which determine the LTI value, aredominated by the DL scheduling delay, which in some systems may be ofthe order of milli-seconds. Similarly, in some systems, the DLscheduling and transmission/propagation times of the ULCS and theULCS-ACK messages, which are also small messages and carry UEidentification and link establishment parameters, may be of the ordermilli-seconds. Accounting for all the delay components, the initialaccess completion time is, in some systems, of the order ofmilli-seconds. Therefore, the UE-to-BS-AOA, estimated by a BS using areceived ULAP from the UE, will not change during the uplink-initiatedinitial access procedure described above, and the said UE-to-BS-AOA maybe used to transmit/receive to/from the UE until the initial access iscompleted.

In one embodiment, the UE is configured with the position locationcoordinates of all the BSs and also estimates its own position locationusing a geo-location technology, the UE uses the position locationinformation to compute the free space path loss from each BS to the UE,and generates a neighbor-set of the BSs whose free space path loss tothe UE is below a threshold. In another embodiment, the network coveragearea is divided into a number of geographic bins, a BS makes a path lossestimate between the BS and a UE in each bin by using the receivedsignal strengths at the BS from the signals transmitted by the UE, andsends the path loss estimates to a CC; in one variation of theembodiment, the BS makes the path loss estimate by dividing the receivedsignal strengths at the BS by the UE's EIRP. For each bin, the CC formsa neighbor-set data base, for each UE in a bin, by listing the BSs whosepath loss estimates to the UE are below a threshold, and includes in thedata base the path loss estimates from the said BSs to the UE. Inanother embodiment, the UE computes a nominal path loss based on afunction of the path loss estimates from the UE to the BSs. Examples ofthe nominal path loss are the path loss from the UE to the nearest BS,or the average of the path losses from the UE to all the BSs in theneighbor-set. In another embodiment, the UE sets the ULAP nominaltransmit power to a value so that the product of the UE's EIRP and theUE to the BS nominal path loss be above a threshold.

In some embodiments, the UE estimates its own position location using ageo-location technology such as a satellite based geo-location system ora terrestrial based network of access points. In one embodiment forinitial access organized by a CC, the UE sequentially transmits a ULAPon the UE beams until the UE receives a ULAP-ACK. Once the UE receives aULAP-ACK, then the UE transmits a message with its position locationcoordinates on the same UE beam on which the ULAP-ACK was received. TheBS that receives the message with the UE's position location coordinatesinforms the CC of the UE's position location coordinates; the CC informsthe BSs in the neighbor-set of the UE's position location coordinatesand requests that the BSs transmit a ULAP-ACK to the UE. Each BS in theneighbor-set computes the UE-to-BS-AoA using the position locations ofthe UE and the BS, forms a DL dedicated control beam toward the UE andtransmits a ULAP-ACK on the said beam. The UE, following thetransmission of the message with its position location coordinates,starts searching for the ULAP-ACKs being transmitted by the BSs in theneighbor-set.

The UE steps for the embodiment for initial access organized by the CCare illustrated in FIG. 6C. In step 102, the UE transmits a ULAP on a UEbeam on which a ULAP has not yet been transmitted, sets the LTI timerand begins the search for ULAP-ACK messages on the same UE beam. In step104, if a ULAP-ACK has been received within the LTI, then in step 202the UE transmits its position location coordinates on the same UE beamon which the UALP-ACK was received, and moves to step 204. In step 204,the UE starts searching, on the other UE beams, for the ULAP-ACKs thatthe BSs in the neighbor-set are transmitting. In step 206, the UEtransmits a ULCS on the UE beam on which the ULAP-ACK with the highestsignal quality is received. If step 104 indicates that a ULAP-ACK hasnot been received within the LTI, the UE moves to step 106. In step 106,if the number of transmissions of the ULAP on the same UE beam is lessthan a number N_(T), then the UE moves to step 108. In step 108, the UEretransmits the ULAP, sets the LTI times and starts the search forULAP-ACKs. In step 106, if the number of transmissions of the ULAP onthe same UE beam is not than a number N_(T), the process moves to step208. In step 208, if ULAPs have not been transmitted on all UE beams,then the UE goes back to step 102 to transmit a ULAP on another UE beam,otherwise the UE goes to step 210 where the process ends. In a variationof the embodiment, once a BS receives the ULCS from the UE, the BSinforms the CC that the UE has completed the DL ULAP-ACK signal qualitymeasurements on all the UE beams, and the CC informs the BSs in the UE'sneighbor-set that they discontinue transmitting the ULAP-ACK to the UE.

The BS with which a UE is currently communicating is referred to as theserving-BS. A BS-serving-beam is referred to as the BS DL data/controlbeam boresight angular pointing position, with respect to a referencedirection, toward a UE with which the BS is communicating. The referencedirection of a BS may be chosen to be the line perpendicular to thecenter of an antenna aperture as shown in FIG. 3, or any other fixedreference direction. A UE-serving-beam is defined as a UE beam boresightangular pointing position toward a BS with which the UE iscommunicating.

In another embodiment for initial access, a UE periodically sends itposition location coordinates to the serving-BS, the serving-BS uses theBS's and UE's position location coordinates to update the UE-to-BS-AoAand the BS-serving-beam, the serving-BS forms a DL dedicated data beamusing the updated BS-serving-beam, the serving-BS transmits a DLDRS onthe updated BS-serving-beam, and the UE detects the DLDRS and carriesout a spatial search of the received DLDRS to update theUE-serving-beam. The accuracy of the position location-based beamtracking scheme depends on the accuracy of the UE's and BS's positionlocation estimates, the BW of the BS-serving-beam, and the periodicitywith which the UE sends its position location updates to the serving-BS.As the serving-BS-beam's BW narrows, higher position location accuracyis required to ensure that the B S-serving-beam accurately points to theUE. The UE position location reporting periodicity needs to be higherthan the time during which the user's mobility may move the UE outsideof the serving-BS-beam's BW. The accuracy of the ULAP based beamtracking depends on the signal quality of the received ULAP afterdispreading and the accuracy of the UL searcher, both of which may bemade very accurate by choosing the length of the ULAP accordingly andusing a high-resolution UL searcher. The overhead of the positionlocation-based beam tracking is higher than that of the ULAP approachbecause a ULAP carries as small as only one bit of information, whereasposition location reporting requires sending more information.

In one embodiment, the UE is equipped with an antenna subsystem capableof forming at least one beam and a radio subsystem that is capable oftransmitting only on one UE beam at a time, the UE radio subsystemtransmits a ULAP on one UE beam at a time and waits for a ULAP-ACKbefore transmitting a ULAP on another beam. In another embodiment, theUE is equipped with an antenna subsystem that is capable of concurrentlyforming multiple beams, and is also equipped with a radio subsystem thatis capable of concurrently transmitting on multiple beams and receivingon multiple beams, the UE concurrently transmits ULAPs on multiplebeams, searches for ULAP-ACKs on the multiple beams, determines thesignal quality of the received ULAP-ACKs, chooses the ULAP-ACK with thehighest signal quality and transmits a ULCS message on the UE beam onwhich the said ULAP-ACK was received. In a variation of the embodiment,the UE stops transmitting ULAPs when a ULAP-ACK with a signal qualityabove a threshold is received, and sends a ULCS on the UE beam on whichthe ULAP-ACK is received.

For fixed UE devices, the UE's antenna beam boresight angular pointingposition may be specified relative to a reference direction, such as aline perpendicular to the center of one the UE's antenna apertures. Inone embodiment, a UE estimates its own position location coordinates,uses the position location coordinates of a BS in the neighbor-set andthat of the UE to compute the BS-to-UE-AoA from the BS to the UE, usesthe said BS-to-UE-AoA to determine an UE antenna aperture and a beamwhich have the highest gain toward the BS, and uses the UE antennaaperture and the beam on which to transmit a ULAP to the BS. FIG. 6D isan exemplary flow chart of the UE steps for an initial access procedure,according to one embodiment. In step 102, the UE transmits a ULAP to aBS in the neighbor-set to which a ULAP has not yet been transmitted,sets the LTI timer and begins the search for ULAP-ACK messages on thesame UE beam. If, in step 104, a ULAP-ACK has been received within theLTI, then the UE in step 202 estimates the signal quality of thereceived ULAP-ACK and moves to step 204. If, in step 104, a ULAP-ACK hasnot been received within the LTI, then the UE goes to step 106. In step106, if the number of transmissions of the ULAP to the same BS is lessthan a number N_(T), then the UE, in step 108, retransmits the ULAP andgoes to step 104, otherwise the UE moves to step 204. In step 204, ifULAPs have not been transmitted to all the BSs in the neighbor-set, thenthe UE goes back to step 102 to transmit a ULAP to another BS, otherwisethe UE goes to step 206, chooses the BS from which the ULAP-ACK with thehighest signal quality is received, and transmits a ULCS to the said BSinitiating a connection setup procedure.

In another embodiment, the initial access for fixed UE devices comprisesof repeating the steps of: the UE transmitting a ULAP signal to one BSat a time in the UE's neighbor-set, the UE retransmitting the ULAP untila ULAP-ACK is received or the number of transmissions of the ULAPreaches a number N_(T), the UE estimating the signal quality of all thereceived ULAP-ACK messages until a ULAP-ACK with a signal quality abovea threshold is received or a ULAP has been sent to all the BSs in theUE's neighbor-set, and the UE sending a ULCS message to the BS fromwhich the ULAP-ACK with the received signal quality above a threshold isreceived.

The ULAPs are expected to generate a small load on the ARACH becausethey carry a small amount of information, as small as one bit accordingto some embodiments, and are also sent only during the initial access.In one embodiment, a small amount of dedicated frequency resources,commensurate with the expected ULAP load, are assigned to the ARACH,wherein the ARACH frequency resources are not assigned to any other ULchannel or UE. In another embodiment, the ULAP may be transmitted as anunderlay signal by spreading the ULAP over a specified frequencyresources, where another UE or UL channel may be scheduled toconcurrently transmit data on the same frequency resources. Forinstance, in an OFDM based system, a UE may transmit a ULAP over aspecified subset of the OFDM tones. The BS despreads the ULAP toaccumulate energy to detect the information carried by the ULAP. Thenumber of tones and OFDM symbols over which the ULAP is spread is chosenso that, even when another UE is transmitting data on the same OFDMtones, the ULAP be detectable with a certain probability of detection,and the ULAP be transmitted at a low power to reduce interference to anydata being transmitted by another UE on the same OFDM tones.

During normal operation, there is a low probability of collision of theULAPs transmitted by multiple UEs because the number of UEs trying toconcurrently carry out initial access is small. Furthermore, since theBSs form narrow UL search beams, then two UEs whose ULAPs collide mustalso be in very close proximity to each other, which further reduces theprobability of ULAP collision. In one embodiment, a number of differentpseudo-noise sequences are used by the UEs to spread their ULAPs, and aUE chooses one of the pseudo-noise sequences, randomly or according to acertain rule, for ULAP transmission. Use of different pseudo-noisesequences increases the probability of detection when multiple ULAPsoverlap in time. In another embodiment, multiple sets of frequencyresources are configured on the ARACH/RACH for ULAP/ULRP transmission,and a UE chooses one of the frequency resources, randomly or accordingto a certain rule, for ULAP transmission. In one embodiment, a UE, whichdoes not receive a ULAP-ACK/ULRP-ACK within the LTI, retransmits theULAP/ULRP with a randomly chosen delay to avoid collision with the ULAPthat another UE may be transmitting.

As was described previously, once a BS detects a ULAP, the BS sends aULAP-ACK on a DL dedicated control beam, formed using the UE-to-BS-AOAat which the ULAP was received. In one embodiment, in an OFDM basedsystem, the ULAP-ACK is transmitted during a DL time slot following thereception of the ULAP, using a subset of the frequency tones and OFDMsymbols of the time slot. In a variation of the embodiment, the frametiming of the DL dedicated control beam transmitting the ULAP-ACK issynchronized to that of the other DL dedicated data beams formed by theBS. If the DL dedicated control beam transmitting the ULAP-ACK overlapswith the DL dedicated data beams of the other UEs, then the DL dedicatedcontrol beam is assigned a set of frequency tones different from thoseof the adjacent beams, thereby avoiding cross-beam interference.

In some embodiments of an uplink-initiated initial access, a UE has notyet detected the DL signal prior to transmitting a first ULAP. Then, theUE does not have an estimate of the path losses to the BSs which willreceive the ULAP transmitted by the UE; a mechanism is needed forsetting the transmit power of the ULAP transmission. In one embodiment,the UE repeats the following procedure: the UE initially transmits aULAP on a UE beam at a ULAP nominal transmit power, waits for an LTItime interval and if no ULAP-ACK is received within the LTI, then the UEincreases its transmit power by a specified 4 and retransmits the ULAPon the same UE beam at the increased power; the UE retransmits the ULAPevery LTI interval by increasing the transmit power by Δ_(P) for eachretransmission until the UE receives a ULAP-ACK or the number of ULAPtransmissions reaches a number N_(T). In a variation of the embodiment,the nominal path loss is set to the smallest UE to BS propagation lossin the UE's neighbor-set. In another variation of the embodiment, theULAP nominal transmit power is set such that the product of the UE'sEIRP and the nominal path loss be above a threshold.

Beam Tracking During Active Data Transmission Mode

As described previously, the current UE-to-BS-AOA is defined as thelatest estimated AoA of the signals received at a BS from a UE. The UEantenna subsystem of mobile and portable devices may change orientationquickly as the user moves the device, thereby changing the UE beamboresight angular pointing position, resulting in the UE beam pointingaway from the serving-BS. Moreover, as the UE's antenna orientationchanges, there may be a large variation in the body loss that the UEantenna beam experiences toward the serving-BS. Even for stationarylaptops, the propagation channel is, due to the movement of people inthe surrounding area, slowly fading. Therefore, systems and methods areneeded to track and update the BS's current UE-to-BS-AOA based on thesignals received from the UE, to track and update the UE-serving-beamand the BS-serving-beam, and to initiate handover when the signalquality received from the serving-BS drops below a threshold.

During a data session, the BS transmits a DL Dedicated Reference Signal(DLDRS) on a DL dedicated data beam toward a UE, which the UE uses fordata demodulation as well as for estimating the DL signal quality. Inone embodiment, the UE transmits a ULRS signal which the serving-BS usesfor data demodulation as well as for estimating the UL signal quality.Once a UE has established data connection with a BS, the BS needs totrack the BS-serving-beam toward the UE as the UE moves or changesorientation. When a BS and a UE are continuously exchanging data, the BSmay estimate the UE-to-BS-AoA by carrying out a spatial search of thereceived ULRS signal quality using a UL search beam around the currentUE-to-BS-AOA, update the current UE-to-BS-AOA based on the UL searchbeam angular pointing position from which the highest signal quality isreceived, form DL and UL dedicated data beams using the updated currentUE-to-BS-AOA and a beamforming algorithm, use the UL dedicated data beamto receive UL data from the UE and to transmit data to the UE on the DLdedicated data beam. In one embodiment, the UE also carries out aspatial search of the received DLDRS signal quality, and updates theUE-serving-beam to a UE beam angular pointing position from which thehighest DLDRS signal quality is received. In another embodiment, the UEinitiates a handover procedure if the signal quality of the receivedDLDRS drops below a threshold. The handover procedure must be completedquickly in order to maintain reliable data transmission continuity withlow delay. Systems and methods for reliable and fast handover aredescribed next.

Beam Tracking During the Idle Mode and the Data Inter-Burst Periods

A UE and its serving BS are in one of two main states, the active statewhen the UE and the serving-BS are exchanging data, and the idle modestate where there is a lull in the data transmission during a datasession. In one embodiment, when a BS in idle mode state has data tosend to a UE, the BS forms a DL dedicated data beam toward the UE usingthe current UE-to-BS-AOA, the BS synchronizes the DL timing of the saidDL dedicated data beam to that of the BS's other DL beams, and sendsDLDRS, DLDCCH and data on the said DL dedicated data beam. The UEsynchronizes to the DLDRS, estimates the channel impulse response forthe allocated frequency resources, and decodes the DL controlinformation and data. In another embodiment, when a UE in idle modestate has data to transmit to a BS, the UE initiates data transmissionon the UE-serving-beam, by either sending data on the RACH or by sendinga resource request message on the RACH to reserve dedicated resources onthe UL on which to transmit data. In one embodiment, during the idlemode state, time is divided into Sleep Time Intervals (STI), and a BS inthe idle mode state that has data to transmit to a UE, starts datatransmission at the STI boundary. In a variation of the embodiment, theUE goes to sleep during idle mode to save power, wakes up every STI tosearch for a DLDRS that the serving BS may be transmitting to start datatransmission, detects the DLDRS, and decodes the DLDCCH and any datasent by the serving-BS. The idle mode ends and active mode begins when aBS or a UE begins data transmission. The STI duration may be chosen tomeet the data transaction delay requirements, while maximizing the UEpower saving. Therefore, data transmission during idle period may beginwith very small delay.

The idle period between data transactions may take seconds or more. Evenduring a data transaction, in some applications such as web browsing,there may be inter-burst delays of 100s of milli-seconds and evenseconds depending on a number of factors such as the server delays. Asmentioned previously, the UE-serving-beam may change direction due tothe user's hand and/or body movement. Even for stationary devices suchas laptops, the UE-serving-beam may experience slow fading due to theuser's movement, or movement of others near the UE. Therefore, duringthe idle periods the UE-serving-beam's gain toward the serving-BS mayreduce significantly if the UE-serving-beam is not adjusted according tothe UE's movements. If the UE has moved out of the BS-serving-beam's BW,then the BS-serving-beam's boresight may not be pointing toward the UE.Therefore, systems and methods are needed to update the BS-serving-beamand UE-serving-beam during the idle mode and the data inter-burstperiods.

In idle mode, a downlink reference signal, referred to as DownLinkTracking Probe (DLTP), is periodically transmitted to the UE by the UE'sserving-BS. The DLTP is transmitted every DLTP Periodicity Time Interval(DLTP-PTI). The UE uses the DLTP to estimate the DL signal quality. Aswill be described in the forthcoming embodiments, if the measured DLTPsignal quality drops below a threshold, then the UE triggers aBS-serving-beam/UE-serving-beam update process, or a handover. In oneembodiment, the DLTP comprises of a preamble that effectively carriesone bit of information, the UE detects the preamble and measures itssignal quality. In one embodiment, the serving-BS assigs a uniquepseudo-noise sequence to the preamble of the DLTP transmitted to eachidle mode UE that is communicating with the BS. In another embodiment,the serving-BS assigns a different time slot for the transmission of theDLTP for each idle mode UE. In another embodiment, in an OFDM basedsystem, the BS assigns a different set of OFDM tones or symbols to theDLTP sent to the different UEs. The PTI duration depends, in part, onthe speed of the UE.

In one embodiment for beam tracking during idle mode, the BSperiodically, at the boundary of DLTP-PTI, forms a DL dedicated controlbeam, using the current UE-to-BS-AoA, toward a UE that is in idle modewith the BS, and transmits a DLTP to the UE. During idle period and datainter-burst periods, the UEs use the DLTP as reference signal for DLsynchronization, DL signal quality measurement and beam tracking. EachUE wakes up every DLTP-TPI, sets a DLTP Wait Time Interval (DLTP-WTI)timer, and searches for the DLTP on the UE-serving-beam. If the UE doesnot detect a DLTP within the DLTP-WTI, the UE initiates the idle modehandover procedure. If the UE detects a DLTP signal within the DLTP-WTI,it estimates the signal quality of the detected DLTP, and initiates ahandover procedure if the estimated DLTP signal quality is below a firstthreshold. If the UE detects a DLTP whose signal quality is below asecond threshold then the UE transmits a ULRP on the UE-serving-beam tothe serving-BS, the serving-BS carries out a spatial search of the ULRPand updates the current UE-to-BS-AOA. In another embodiment, the UE usesdigital beamforming, stores the received DLTP samples in a memorybuffer, does a spatial search of the DLTP samples to determine the UEbeam angular pointing position at which the UE receives the DLTP withthe highest signal quality, and chooses the said UE beam angularpointing position as the UE-serving-beam. The idle mode handoverprocedure follows the same steps as those of the handover proceduresdescribed in the previous embodiments.

In some embodiments, the UE is equipped with a UE positioning subsystemcomprised of sensors such as accelerometers, gyroscopes andmagnetometers. The UE positioning subsystem tracks the changes in theUE's orientation and position location coordinates with respect to areference time. In one embodiment, the UE positioning subsystem tracksthe changes in the UE orientation and position location coordinates,computes a metric of the changes in the UE's orientation and positionlocation, and triggers a beam update process when the magnitude of themetric of the changes in the UE orientation or position location is aabove a threshold. In a variation of the embodiment, the UE radiosubsystem goes into sleep mode during idle periods, the UE positioningsubsystem triggers the UE radio subsystem to wake up and begin a beamupdate process when the magnitude of the metric of the changes in the UEorientation or position location is a above a threshold. Examples ofmetrics of the UE orientation change are the sum of the changes in theazimuthal and elevation angles, and the maximum of the azimuthal orelevation angle changes. Examples of metrics of the UE position locationcoordinate change are the length of UE position location change, and thechange in the horizontal position change. In another embodiment for idlemode beam tracking, in addition to the UE orientation and positionlocation change based beam update trigger, a DLTP is also sent to the UEby the serving-BS every DPLS-PTI.

In a UE positioning change based beam update embodiment, once the UE'sorientation and position location changes trigger a beam update process,the UE begins the beam update process by sending a ULRP signal to the BSon the UE-serving-beam. The BS beam update process comprises of the BSsearching for the ULRP, the BS carrying out a spatial search of thedigital samples of a detected ULRP signal to determine the AoA at whichthe BS receives the ULRP with the highest signal quality, the BSupdating the current UE-to-BS-AoA to the determined AoA, and the BSsending a ULRP-ACK to the UE on the BS-serving-beam. FIG. 9 is anexemplary flow chart of the UE steps of the UE positioning change basedbeam update embodiment. In step 102, the UE transmits a ULRP and setsthe LTI timer, and begins the search for the ULRP-ACK. In step 104, if aULRP-ACK has been received within the LTI, then the UE goes to step 106,otherwise the UE goes to step 202. In step 106, the UE initiateshandover if the signal quality of the received ULRP-ACK is below a firstthreshold, otherwise the UE initiates the UE-serving-beam update processif the signal quality is below a second threshold. In step 202, if thenumber of transmissions of the ULRP exceeds a number N_(T), then the UEinitiates handover, otherwise the UE goes back to step 102. During theUE-serving-beam update process, the UE carries out a spatial search ofthe received digital samples of the ULRP-ACK message to determine the UEbeam angular pointing position at which the ULRP-ACK is received withthe highest signal quality and updates the UE-serving-beam angularpointing position to that of the determined UE beam angular pointingposition.

Fast and Low Overhead Handover

During the active and idle modes, the BS transmits DLDRS and DLTPreference signals, on DL dedicated control and data beams, to the UE.The UE estimates the signal quality of the DLDRS and DLTP, and if thesignal quality is below a threshold then the UE initiates handover. Inone uplink-initiated handover embodiment, the UE steps comprise of theUE sequentially transmitting a UL reference signal on each UE beam,estimating the signal quality of the acknowledgements received from theBSs in response to the transmitted UL reference signals, and choosing,as handover-BS, a BS from which an acknowledgement with signal qualityabove a threshold is received. During the initial access and datasession, the UE has synchronized to the DL, has decoded a DLDCCH, andhas received information on the RACH resources of the UE's serving-BS aswell as the BSs in the UE's handover-candidate-set. Then, duringhandover, the UE may transmit a UL reference signal on the RACH,referred to as the UpLink Handover Probe (ULHP). The ULHP includes thefunctionality of the ULAP. In some embodiments, the ULHP comprises of asingle preamble that is spread using a modulation symbol pattern, orpseudo-noise sequence. In some embodiments, the ULHP comprises of apreamble and a payload carrying information such as the UE'sidentification. In some embodiments, the ULHP-ACK, the acknowledgementthat the BS sends to the UE when a ULHP is received, comprises of asingle preamble, effectively carrying one bit of information. In someembodiments, the ULHP preamble is spread using a BS specificpseudo-noise sequence. In some embodiments, the ULHP-ACK is comprised ofa preamble and a payload that includes information such as the BS'sidentification.

FIG. 8 is an exemplary flow chart of the UE steps for anuplink-initiated handover procedure, according to one embodiment. Instep 102, the UE transmits a ULHP on a beam on which a ULHP has not yetbeen transmitted, sets the LTI timer and begins the search for ULHP-ACKmessages on the same UE beam. In step 104, the UE determines if aULHP-ACK has been received within the LTI. If a ULHP-ACK is receivedwithin the LTI, then the UE, in step 202, estimates the signal qualityof the received ULHP-ACK and goes to step 204. If a ULHP-ACK is notreceived within the LTI, then the UE, in step 106, determines if thenumber of the ULHP transmissions is below a number N_(T); If the numberof the ULHP transmissions is below N_(T), then the UE retransmits theULHP, sets the LTI, and begins the search for the ULHP-ACK and goes backto step 104, otherwise the UE moves to step 204. In step 204, if ULHPshave not been transmitted on all UE beams, then the UE goes back to step102, otherwise the UE, in step 206, chooses the UE beam on which it hasreceived the ULHP-ACK with the highest signal quality and transmits anUpLink Handover Message (ULHM) on the said UE beam, requesting handover.In a variation of the embodiment, the ULHP-ACK carries the BS'sidentification, the UE designates the BS from which the ULHP-ACK withthe highest signal quality is received as the handover-BS, and sends aULHM to the serving-BS, requesting that the serving-BS complete thehandover between the UE and the handover-BS. In another variation of theembodiment, the ULHP-ACK carries the BS's identification, the UE sendsthe identity of each BS from which it has received a ULHP-ACK as well asthe signal quality of each received ULHP-ACK to the serving-BS,requesting that the serving-BS, or the CC, choose a handover-BS andcomplete the handover to the handover-BS.

In another handover embodiment, the UE transmits a ULHP on a UE beam,sets the LTI timer and begins the search for the ULHP-ACK on the same UEbeam. If a ULHP-ACK is not received within the LTI then the UEretransmits the ULHP on the same UE beam if the number of the ULHPtransmissions is below a number N_(T). If a ULHP-ACK is received withinthe LTI, then the UE estimates the signal quality of the receivedULHP-ACK, and transmits a ULHM message if the signal quality is above athreshold, otherwise if ULHPs have not been transmitted on all UEs beamsthen the UE goes back to the step of transmitting a ULHP on another UEbeam. In a variation of the embodiment, the ULHP-ACK carries the BS'sidentification, and the UE chooses, as the handover-BS, the BS fromwhich it has received a ULHP-ACK with a signal quality above athreshold. In another variation of the embodiment, the UE transmits theULHM to the handover-BS on the same UE beam on which the ULHP-ACK withthe signal quality above a threshold was detected, requesting that thehandover-BS complete the handover. In another variation of theembodiment, the UE transmits the ULHM, which contains the identity ofthe handover-BS, on the UE-serving-beam to the serving-BS, requestingthat the serving-BS complete the handover to the handover-BS.

In another handover embodiment, the UE sends a message to the serving-BSthat includes the UE's position location coordinates and informs theserving-BS that the handover procedure has been initiated, theserving-BS forwards the message to the CC, and the CC requests that theBSs in the UE's neighbor-set transmit a ULHP-ACK to the UE. Each BS inthe neighbor-set computes the UE-to-BS-AoA using the position locationsof the UE and the BSs, forms a DL dedicated control beam toward the UEusing the UE-to-BS-AoA, and transmits a ULHP-ACK on the DL dedicatedcontrol beam. The UE, searches for ULHP-ACKs on all its beams, estimatesthe signal quality of the received ULHP-ACKs, chooses the UE beam onwhich the ULHP-ACK with the highest signal quality is received, andsends a ULHM on the said UE beam requesting handover. In a variation ofthe embodiment, the ULHP-ACKs carry information regarding the BS'sidentification, the UE chooses the BS from which it has received theULHP-ACK with the highest signal quality as the handover-BS, and the UEsends a ULHM to the serving-BS including the identity of thehandover-BS, requesting handover to the handover-BS. In a variation ofthe embodiment, the UE sends the ULHM to the handover-BS requestinghandover. In another variation of the embodiment, the UE sends a messageto the serving-BS with the signal quality estimate of the ULHP-ACKsreceived from the BSs in the handover-candidate-set; the serving-BS, orthe CC, determines the handover-BS and completes the handover process.In another variation of the embodiment, the BS that completes thehandover informs the CC of the handover completion, and the CC sends amessage to each BS in the neighbor-set requesting that the BS stoptransmitting the ULHP-ACK.

In one handover embodiment for fixed UE devices, the UE in a first steptransmits a ULHP to a BS in the handover-candidate-set to which a ULHPhas not yet been transmitted, sets the LTI timer and begins the searchfor the ULHP-ACK on the same UE beam on which the ULHP was sent. If aULHP-ACK is not received within the LTI, then if the number of the ULHPtransmissions is below a number N_(T) the UE retransmits the ULHP to thesame BS. If a ULHP-ACK is received, then the UE estimates the signalquality of the received ULHP-ACK. Next, the UE goes back to the firststep of transmitting a ULHP to another BS in the handover-candidate-setif ULHPs have not been transmitted to all the BSs in thehandover-candidate-set, otherwise the UE chooses, as handover-BS, the BSfrom which it has received the ULHP-ACK with the highest signal quality,and transmits a ULHM message to the handover-BS requesting that the saidBS complete the handover. In a variation of the embodiment, the UE sendsa ULHM message to the serving-BS, using the UE-serving-beam, requestingthat the serving-BS complete the handover. In another variation of theembodiment, the UE sends a message to the serving-BS with the signalquality estimate of the ULHP-ACKs received from the BSs in thehandover-candidate-set; the serving-BS, or the CC, determines thehandover-BS and completes the handover process.

What is claimed is:
 1. A system for handover of a UE in a network ofBSs, the UE and BSs are equipped with at least one antenna aperture andat least one radio sub-system, and are capable of forming at least onebeam, wherein: a BS transmits a dedicated downlink reference signal to aUE with which the BS is communicating; and the UE measures a signalquality of the dedicated downlink reference signal, and if the signalquality is below a threshold then the UE and all BSs repeat: the UEtransmits an uplink reference signal on a UE beam; the BSs search forthe uplink reference signal; each BS estimates an Angle of Arrival (AoA)of a detected uplink reference signal, forms a dedicated downlink beamtoward the AoA, and sends an acknowledgement on the dedicated downlinkbeam; and the UE measures the signal quality of the acknowledgement;until the uplink reference signal has been transmitted on each UE beam.2. System of claim 1, wherein the UE: chooses a UE beam from which anacknowledgement with a highest signal quality is received; and sends ahandover message on the said UE beam.
 3. System of claim 1, wherein theacknowledgement comprises of a preamble, spread using a pseudo-noisesequence.
 4. System of claim 1, wherein the acknowledgement carries theBS's identification.
 5. System of claim 4, wherein the UE: chooses a BSfrom which an acknowledgement with a highest signal quality is receivedas a handover BS; and sends a handover message, requesting handover tothe handover BS.
 6. System of claim 4, wherein: the UE forms a handovercandidate set of all BSs from which an acknowledgement with a signalquality above a threshold is received, and sends the handover candidateset to a central controller; and the central controller chooses from thehandover candidate BS a handover BS to which to handover.
 7. System ofclaim 1, wherein the uplink reference signal is comprised of a preamble,spread using a pseudo-noise sequence.
 8. System of claim 1, wherein theBS periodically transmits the dedicated downlink reference signal. 9.System of claim 8, wherein the UE is in sleep mode between periodictransmissions of the dedicated downlink reference signal.
 10. System ofclaim 1, wherein the UE determines a UE beam angular pointing positionon which the dedicated downlink reference signal is received with ahighest signal quality, and chooses the UE beam angular pointingposition as a UE beam boresight.
 11. A system for handover of a UE,wherein: a BS transmits a dedicated downlink reference signal to the UE;and the UE measures a signal quality of the dedicated downlink referencesignal, and if the signal quality is below a threshold then the UE andall BSs repeat: the UE transmits an uplink reference signal on a UEbeam; the BSs search for the uplink reference signal; each BS estimatesan Angle of Arrival (AoA) of a detected uplink reference signal, forms adedicated downlink beam toward the AoA, and sends an acknowledgement onthe dedicated downlink beam; and the UE measures the signal quality ofthe acknowledgement; until the uplink reference signal has beentransmitted on each UE beam or an acknowledgement with a signal qualityabove a threshold is received.
 12. System of claim 11, wherein the UEsends a handover message on a UE beam on which an acknowledgement with asignal quality above a threshold is received.
 13. A method for handoverof a UE between BSs, comprising: a BS transmitting a dedicated downlinkreference signal to the UE; and the UE measuring a signal quality of thededicated downlink reference signal, and if the signal quality is belowa threshold then the UE and all BSs repeating: the UE transmitting anuplink reference signal on a UE beam; the BSs searching for uplinkreference signal signals; each BS determining an Angle of Arrival (AoA)of a detected uplink reference signal, forming a dedicated downlink beamtoward the estimated AoA, and sending an acknowledgement on thededicated downlink beam; and the UE measuring the signal quality of theacknowledgements; until the uplink reference signal has been transmittedon each UE beam.
 14. Method of claim 13, further comprising, the BSincluding the BS's identification in the acknowledgement.
 15. Method ofclaim 14, further comprising: the UE choosing a BS from which anacknowledgement with a highest signal quality is received as a handoverBS; and sending a handover request message, requesting handover to thehandover BS.
 16. Method of claim 14, further comprising: the UE forminga BS handover candidate set of all BSs from which an acknowledgementwith a signal quality above a threshold is received; sending the BShandover candidate set to a BS with which the UE is communicating; andthe BS choosing a handover BS from the handover candidate BS, to whichto handover the UE.
 17. Method of claim 13, sending the acknowledgementcomprising of a single reference signal.
 18. Method of claim 13, furthercomprising, the BS periodically transmitting the dedicated downlinkreference signal.
 19. Method of claim 18, further comprising, the UEbeing in sleep mode between periodic transmissions of the dedicateddownlink reference signal.
 20. Method of claim 13, further comprising,the UE determining a UE beam angular pointing position on which thededicated downlink reference signal is received with a highest signalquality, and choosing the UE beam angular pointing position as a UE beamboresight.